1. Field of the Invention
This invention relates to a radiation image read-out and erasing method for reading out a radiation image stored on a stimulable phosphor sheet and erasing the residual image information remaining on the stimulable phosphor sheet and to an apparatus for carrying out the method.
2. Description of the Related Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as "a stimulable phosphor".
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a recording medium provided with a layer of the stimulable phosphor. Though the recording medium may be in various forms such as a sheet, a panel, a drum and the like, it will be referred to as "a stimulable phosphor sheet", hereinbelow. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image suitable for diagnosis. See Japanese Unexamined Patent Publication No. 56(1981)-11395 and U.S. Pat. Nos. 4,258,264, 4,315,318, 4,387,428, and 4,276,473.
In such a system, it is said that it is preferred to use light rays within the wavelength range of 600 to 700 nm as the stimulating rays and to detect light emitted by the stimulable phosphor sheet upon stimulation thereof within the wavelength range of 300 to 500 nm in order to separate the stimulating rays from the light emitted by the stimulable phosphor sheet upon stimulation thereof so that even feeble emission can be effectively detected. It is further said that stimulable phosphors which emit light within the wavelength range of 300 to 500 nm upon stimulation by stimulating rays within the wavelength range of 600 to 700 nm are preferably used. (See U.S. Pat. No. 4,258,264.)
From the economical viewpoint, it is preferred that the stimulable phosphor sheet be repeatedly used.
Though the radiation energy stored on the stimulable phosphor sheet during exposure to radiation passing through an object should be released from the stimulable phosphor sheet when the intensity of the stimulating light rays to which the stimulable phosphor sheet is exposed during read-out of the radiation image is sufficient, actually the radiation energy stored on the stimulable phosphor sheet cannot be completely released by only the stimulating light rays, and a part of the radiation energy is kept on the stimulable phosphor sheet as residual radiation energy. The residual radiation energy can appear as noise in a radiation image taken next when the stimulable phosphor sheet is reused.
Further since a fine amount of radioelements such as .sup.226 Ra, .sup.40 K and the like are contained in the stimulable phosphors, the stimulable phosphor sheet stores thereon radiation energy of radiation from the radioelements even if it is left to stand, and the radiation energy thus stored can cause noise. Further energy of environmental radiations such as cosmic radiations, radiations from radioactive isotopes in the environment and the like can be stored on the stimulable phosphor sheet. Such radiation energy stored on the stimulable phosphor sheet while the stimulable phosphor sheet is left to stand appears as fogging in a radiation image taken next. This applicant has proposed a method of causing a stimulable phosphor sheet to sufficiently release radiation energy stored thereon in order to prevent fogging due to radiation energy stored while the stimulable phosphor sheet is left to stand and noise due to residual radiation energy from appearing in a radiation image subsequently taken by exposing the stimulable phosphor sheet to erasing light containing therein light component having a wavelength within the stimulating wavelength range of the stimulable phosphors before recording a radiation image on the stimulable phosphor sheet.
As such erasing methods, there have been known those in which a light source emitting light of a relatively long wavelength such as a tungsten lamp which emits visible light or infrared radiation, a halogen lamp or an infrared lamp is employed as the erasing light source (U.S. Pat. No. 4,400,619), in which a light source emitting light of a relatively short wavelength (about 400 to 600 nm) such as a fluorescent tube, a laser, a Na lamp, a Ne lamp, a metal halide lamp or a Xe lamp is employed as the erasing light source (U.S. Pat. No. 4,496,838), and in which the stimulable phosphor sheet is exposed to erasing light twice, the second erasing being carried out just before the stimulable phosphor sheet is reused by exposing the stimulable phosphor sheet to erasing light which is 1/5 to 3/10000 of that used in the first erasing in amount (U.S. Pat. No. 4,439,682). It is said that erasure can be most efficiently effected when visible light is employed as the erasing light.
However when the erasing light contains no wavelength in the ultraviolet region, residual radiation energy carried by trapped electrons at a relatively deep level which is hard to release by visible light cannot be sufficiently erased. On the other hand, when the erasing light contains a large amount of wavelengths in the ultraviolet region, new trapped electrons are formed by the erasing light in the ultraviolet region though the residual radiation energy carried by trapped electrons at a relatively deep level can be erased.
Thus it has been very difficult to efficiently erase radiation energy carried by both the normal trapped electrons and trapped electrons at a relatively deep level, and accordingly, for instance, when a high-sensitive radiation image recording is carried out, the residual radiation energy affects the quality of the image. At present, in order to overcome such a problem, the short wavelength component of the erasing light has to be finely controlled. Thus there has been proposed a method of erasing residual radiation energy in which the stimulable phosphor sheet is first erased by erasing light containing a wavelength component in the ultraviolet region and then erased by erasing light having a wavelength longer than the ultraviolet region so that radiation energy carried by trapped electrons at a deep level can be erased as well as radiation energy carried by trapped electrons at normal level. (U.S. Pat. No. 5,065,021)
There also has been proposed a simultaneous read-out and erasing apparatus in which a read-out section having a photoelectric read-out means for reading out a radiation image on the stimulable phosphor sheet is disposed adjacent to an erasing section for erasing residual radiation energy on the stimulable phosphor sheet and while the radiation image on the stimulable phosphor sheet is being read out from one end to the other end of the stimulable phosphor sheet, erasing light is projected onto the stimulable phosphor sheet from said one end to the other following the read-out operation. See, for instance, Japanese Unexamined Patent Publication No. 4(1992)-32046. With this apparatus, read-out and erasure can be efficiently carried out in a short time.
However an attempt to employ the erasing method disclosed in U.S. Pat. No. 5,065,021 in carrying out simultaneous read-out and erasing will encounter the following difficulties. That is, when erasing light in the ultraviolet region is employed in the erasing operation following the read-out operation, the erasing light can be detected by the photoelectric read-out means to generate noise in a reproduced radiation image since the wavelength range of the light emitted from the stimulable phosphor sheet upon stimulation thereof is relatively close to the ultraviolet region.
Generally the photoelectric read-out means is provided with a stimulating ray cut filter which cuts the stimulating rays and accordingly when light of a wavelength to be cut by the stimulating ray cut filter is employed as the erasing light, the erasing light cannot generate noise in a reproduced radiation image.
However when the light of a wavelength to be cut by the stimulating ray cut filter is employed as the erasing light, erasing efficiency is low and the residual radiation energy cannot be sufficiently erased.
Thus in the simultaneous read-out and erasing system, the residual radiation energy cannot be sufficiently erased and noise is generated in a reproduced radiation image, which results in deterioration in quality of the reproduced radiation image.